1. Field of the Invention
The present invention relates to an image forming method using a developing device employing a roll magnet having no sleeve on its outer peripheral as a developer conveying member.
2. Description of the Prior Art
Conventionally known image forming devices include copying machines, printers, and facsimile terminal equipment. Among these devices, those using an electrophotograhic or electrostatic recording method generally supply a developer from developer conveying member provided in proximity to a image-bearing member to deposite the toner in the developer onto electrostatic latent images formed on the surface of the image-bearing member by means of light exposure to form image.
The image-bearing member and the developer conveying member are opposed at a specified gap. The main part of the developer conveying member mainly comprises a roll-like magnet for conveying a developer which has on its outer perifheral a sleeve made of a non-magnetic material (hereinafter referred to as a roll magnet), a magnetic brush regulating member for regulating the developer held on the surface of the sleeve to a specified layer thickness (hereinafter referred to as a doctor blade).
The roll magnet comprises a roll-like magnet provided in a developer hopper for storing the developer and having a plurality of magnetic poles on its surface and a sleeve covering the surface of the magnet so that the sleeve can rotate relative to the magnet. The roll magnet and the image-bearing member on which electrostatic latent images are to be formed are opposed to each other at a small specified gap.
While attracted to and retained on the surface of the sleeve, the developer passes through the very small gap between the surface of the sleeve and the doctor blade, when it is formed as a thin layer. The developer is then conveyed to the developing region where the roll magnet and the image-bearing member are opposed.
In response to an increased number of requirements, efforts are being made to improve image quality and to reduce the cost and size of image forming devices. Under these circumstances, a variety of proposals has been presented as candidates for a developing device that is the main component of the image forming device. For example, the use of a sleeveless type roll magnet but without the sleeve for retaining the developer around a magnet to develop electrostatic latent images has been proposed (for example, GB2150465A, PUPA 63-223675, and PUPA 62-201463). Developing devices using such a sleeveless type roll magnet usually use a single-component developer (magnetic toner) because it reduces the size of these devices and eliminates the need for maintenance.
However, methods using a sleeveless type roll magnet and a single-component developer do not apply a sufficient amount of charge to the toner constituting the developer, resulting in poor image quality. The doctor blade is thus pressed against the surface of the roll magnet to pass toner particles between the roll magnet and the doctor blade pressed against it, thereby causing triboelectric charge to the toner, This also enables toner retained on the surface of the roll magnet to become a thin layer.
Even if the doctor blade is pressed against the surface of the roll magnet, however, a sufficient amount of toner not be attracted on the surface of the sleeveless type roll magnet if a small amount of triboelectric charge is applied to the toner. To resolve this problem, fine electrodes have been proposed to be provided on the surface of the sleeveless type roll magnet attract toner (see GB2150465A).
This structure requires, however, that new fine electrodes be provided on the surface of the roll magnet, which fails to achieve the initial object of removing the sleeve to simplify the structure of the roll magnet and thereby to reduce the cost and size of the device.
On the one hand, a bias voltage (including a ground) must be applied to the developer to prevent fogging and to obtain reverse images in the developing region. To do this, the application of conductivity at least to the surface of the roll magnet has been proposed (see PUPA 62-201463 referenced above). Although this structure reduces cost, however, it still remains to be improved because the manufacture of such a roll magnet takes time.
On the other hand, the magnetic brush method is likely to cause fogging because a magnetic brush comprising a magnetic developer slides contact not only the image area forming electrostatic latent images but also the nonimage area.
A developing method based on jumping developing has thus become known; in this method, the layer of magnetic developer having a thickness less than the gap between the surface of the image-bearing member and the surface of the sleeve in the developing region is used to convey toner in a magnetic developer to electrostatic latent images (for example, see U.S. Pat. No. 4,292,387 or U.S. Pat. No. 4,342,822). In this jumping developing method, an AC bias voltage applied in the developing region to improve the reproductivity (gradient) of half-tones.
FIG. 5 is a transverse cross section of the important part of the prior art showing an example of the jumping developing method. In this figure, reference numeral 1 designates the developer vessel 1 accommodating the magnetic developer 2. Provided below the magnetic developer is a developing roller 6 comprising a permanent magnet member 4 comprising a plurality of permanent magnets 3 and shaped in the form of a cylinder and a sleeve 5 made of a non-magnetic metal material such as a austenic stainless steel (for example, SUS304) in such a way that the permanent magnet member and the sleeve are coaxial and rotatable relative to each other.
Reference numeral 7 is a drum having a photosensitive (photosemiconduction) layer on its surface formed so as to rotate in the direction of the arrow and opposed to the developing roller 6 with a gap (g) in between. Reference numeral 8 is a doctor blade provided on the developer vessel 1 and opposed to the developing roller 6 with a gap (t) in between for regulating the thickness of the layer of magnetic developer attracted onto the sleeve 5 constituting the developing roller 6. Reference numeral 9 is an AC power supply connected between the photosensitive drum 7 and the doctor blade 8 to apply an AC bias voltage. The gap (g) is set to be greater than the thickness of the layer of the magnetic developer on the sleeve 5.
With the above structure, when the permanent magnet member 4 is fixed and the sleeve 5 is rotated in the direction of the arrow, the magnetic developer 2 is attracted to the sleeve 5 and then conveyed. The developer arrives in the developing region opposite to the photosensitive drum 7, where the electric fields of electrostatic latent images formed on the photosensitive drum 7 cause the magnetic developer 2 to overcome the attractive force of the permanent magnet member 4 to sleeve 5 to move onto the drum 7. This enables electrostatic latent images to be developed.
In the jumping developing method described above, the thickness of the layer of magnetic developer 2 on the sleeve 5 is generally less than 0.2 to 0.4 mm as is common with ordinary magnetic brush developing methods, for example, about 0.1 mm. The roundness of sleev 5 and concentricity of the outer circumferential surface of the permanent magnet member 4 and sleeve 5 must be improved to perform highly accurate machining.
The outer circumferential surface of the sleeve 5 attracts the magnetic developer 2 using the magnetic attractive force of the permanent magnet member 4, and conveys it using frictional force. To improve conveying abikity, for example, blasting is usually performed to make the roughened surface of the sleeve. However, since friction progresses during operation to change the coefficient of friction or to cause other local changes, the thickness of the layer of magnetic developer 2 that is attracted is changed in such a way as to adversely affect developing. A slight temporal change in the conditions of the surface of the sleeve 5 severely and adversely affects developing because the thickness of the layer of the magnetic developer on the sleeve 5 is small in the jumping developing method as described above.
Methods for using the magnetic brush method to develop electrostatic latent images by removing the sleeve 5 constituting the developing roller 6 and using the permanent magnet member 4 alone have also been proposed to miniaturize printers as described above (for example, see PUPA 62-201463). In these methods, about half the height of the magnetic brush comprising the magnetic developer contacts the surface of the photosensitive drum 7
In this form of magnetic brush method, however, the insufficient accuracy of the permanent magnet member 4 causes deflection resulting in nonuniform images. Thus, when the gap between the surface of the photosensitive drum 7 and the surface of the permanent magnet member 4 in the developing region, that is, the developing gap is widened, the gap between the surface of the doctor blade 8 and the surface of the permanent magnet member 4, that is, the doctor blade gap, must be widened accordingly. However, a large doctor blade gap prevents sufficient triboelectric charge from being applied to the toner in the magnetic developer 2, thereby causing frequent fogging.